Ming-Jyh Chern

Control of Volumetric Flow-Rate of Ball Valve Using V-Port

The control of volume flow rate in a ball valve is very important when a ball valve is utilized in a piping system. It is difficult to linearly control the flow rate in a ball valve without external devices. V-ports are employed to achieve this purpose. In order to investigate the effects of V-port on the volume flow rate and flow features, 3-D numerical simulations and experiments were conducted to observe the flow patterns and to measure performance coefficients when V-ports with various angles were used in a piping system. Three V-ports with angles 30 deg, 60 deg, and 90 deg were studied

Numerical Study on Cavitation Occurrence in Globe Valve

AbstractCavitation in a valve leads to trouble and inconvenience for factories. Valves in a piping system are ruined, leading to costly replacements every several months. To reduce the cost caused by cavitation in a valve, a cage is utilized to make cavitation occur only in the region adjacent to the cage itself; therefore, only the cage needs to be replaced. To validate the design of a cage, simulation of the turbulent flow field inside a globe valve and the occurrence of cavitation are necessary for a valve designer. To reach this purpose, prediction of the cavitation inside the globe valve with and without a cage is undertaken, and a cavitation model is established in this study. The percentage of vapors in each computational cell is calculated using the proposed cavitation model. Two various cages, the one-stage perforated cage and the one-stage step cage, are considered. Vapor resulting from cavitation appears in the vortices existing inside the valve and at the downstream region of the globe valve w...

Effect of Corrugated Bed on Hydraulic Jump Characteristic Using SPH Method

AbstractA hydraulic jump is a common phenomenon that can be observed in open channel flows such as rivers and spillways. It can cause damage of the downstream bed and bank of the channel due to the process of continuous erosion and degradation. In order to reduce the hydraulic jump destruction, the energy in the hydraulic jump must be dissipated as much as possible. One method to increase dissipation of energy is using a corrugated bed. In order to know the effect of a corrugated bed on the hydraulic jump, a smoothed particle hydrodynamics (SPH) model is applied to investigate the characteristics of hydraulic jumps in various corrugated beds. A variety of corrugated beds that are smooth, triangular, trapezoidal, and sinusoidal are considered. The opening of a gate is changed to adjust the hydraulic jump. The conjugate depth ratio, the jump length, the bottom shear stress distribution, and the energy dissipation are reported. The results of the present study are in a good agreement with previous studies. E...

Numerical investigation of regurgitation phenomena in pulmonary arteries of Tetralogy of Fallot patients after repair

Pulmonary regurgitation is a very common phenomenon in pulmonary arteries after repair of patients of Tetralogy of Fallot (TOF) which is the most common complex congenital heart diseases. The aim of this study is to use numerical approaches to simulate flow variations in pulmonary artery after repair of patients of TOF. We analyze the flow patterns in an in-vitro bifurcation pulmonary artery and consider effects of various regurgitation fractions (RF or b/f) in left pulmonary artery (LPA) and right pulmonary artery (RPA). We not only observe the variation of flow patterns, but also analyze the results of b/f and net volumetric flow rates in LPA and RPA. In general, the b/f of LPA is higher than RPA in the measured data provided by phase-contrast magnetic resonance imaging (PC-MRI). We validate the result using numerical approaches to analyze the flow patterns in pulmonary artery in this study. The results will be useful for medical doctors when they perform operations for TOF patients.

Numerical Investigation and Recommendations for Push-Pull Ventilation Systems

This study presents numerical simulations of push-pull ventilation systems. A push-pull system is a device commonly used in capturing pollutants from large tanks used in industrial chemical processes. An air jet is blown from one side of a tank and collected by an exhaust hood on the opposite side of the tank. In this study, a finite volume model coupled with the standard k −ϵ turbulent model is employed to describe the flow structures and characteristics. Moreover, the turbulence mass transfer equation is adopted to show the concentration distribution above the open surface tank. All the flow fields can be classified according to four dominant modes, i.e., dispersion, transition, encapsulation, and strong suction. The push and pull flow velocities should be adjusted into encapsulation and strong suction modes to ensure all pollutants can be captured by the exhaust hood. Other geometric parameters such as the flange size, pull-channel size, offset distance, etc., also influence the flow characteristics. For a variety of lengths of tanks and pollutant evaporation velocities, the push and pull flow velocity must be matched to achieve optimal operation. Furthermore, the flange size and other parameters are determined to enhance the capture efficiency of the push-pull system. Recommendations for design guidelines are introduced in this study.

Interaction of Oscillatory Flows with a Square Cylinder

The behaviour of vortices induced by a single square cylinder in an oscillating flow was investigated. The flow patterns in the vicinity of square cylinders were visualized using an in-house numerical model. Meanwhile, force coefficients exerted on the square cylinder were determined numerically. In terms of various Keulegan-Carpenter (KC) numbers, it turns out that the flow patterns for an oscillating flow past a single cylinder can be divided into three modes: (i) no vortex, (ii) pairs of symmetric vortices, and (iii) asymmetric vortex shedding. Reynolds (Re) number did not affect the flow field apparently in this study. In addition, the in-line force coefficient decreases exponentially as KC increases. Spectrum analysis of in-line force coefficients for various KCs was provided. It can be found that the flow system was at a periodic state at small KC for the first two modes. Variations of the flow system from a periodic state to a highly nonlinear state in which asymmetric vortex shedding appeared were demonstrated for increasing KC. The relationship between the in-line force and KC was provided for future applications.

Design of cages in globe valve

A globe valve is a common device that performs flow control in a pipe system. The flow capability of the globe valve is estimated by a flow coefficient. To improve the control capability of the globe valve, a cage is utilized to adjust the flow coefficient in the valve. For example, if one feels like decreasing the flow coefficient, then a cage with numerous passageways will be considered rather than replacing a smaller valve and pipe. A double cages design is also employed to reach the same purpose. The flow coefficient of the valve is associated with passageways in the cage. Provided that the relationship between the total cross-sectional area of passageways and the flow coefficient is known, then one will be able to design another cage with required and re-allocated passageways to control the variation of flow coefficient. The objective of this study is to use computational fluid dynamic technique to predict the relationship. Furthermore, one can change variation of flow coefficient in a globe valve using a new designed cage with re-allocated passageways. Again, the change of the cage on the flow coefficient can be validated by computational fluid dynamics again. This approach would be useful for a valve engineer to design a cage to control the flow capability in a globe valve.

Numerical Prediction of Hydrodynamic Loading on Circular Cylinder Array in Oscillatory Flow Using Direct-Forcing Immersed Boundary Method

Cylindrical structures are commonly used in offshore engineering, for example, a tension-leg platform (TLP). Prediction of hydrodynamic loadings on those cylindrical structures is one of important issues in design of those marine structures. This study aims to provide a numerical model to simulate fluid-structure interaction around the cylindrical structures and to estimate those loadings using the direct-forcing immersed boundary method. Oscillatory flows are considered to simulate the flow caused by progressive waves in shallow water. Virtual forces due to the existence of those cylindrical structures are distributed in the fluid domain in the established immersed boundary model. As a results, influence of the marine structure on the fluid flow is included in the model. Furthermore, hydrodynamic loadings exerted on the marine structure are determined by the integral of virtual forces according to Newton’s third law. A square array of four cylinders is considered as the marine structure in this study. Time histories of inline and lift coefficients are provided in the numerical study. The proposed approach can be useful for scientists and engineers who would like to understand the interaction of the oscillatory flow with the cylinder array or to estimate hydrodynamic loading on the array of cylinders.

Effect of plasma actuator in boundary layer on flat plate model with turbulent promoter

This paper shows experimental results for velocity measurement in the boundary layer with the use of a flat plate model. The flat plate model is disrupted with a wire trip and the effect of the plasma actuator to alter the flow in the boundary layer is then observed. The purpose of this research is to characterize the performance of the plasma actuator in a no-flow condition and with the use of a 2 m/s flow and also to theoretically analyze the performance of actuator in the boundary layer namely, displacement thickness, momentum thickness, and energy thickness. This is all done to acquire a deeper understanding of the capabilities of plasma actuator as one of the alternative active flow control equipment and to increase the effect of aerodynamic drag reduction. One of the ways to decrease the aerodynamic drag is to manipulate the flow to have a low boundary layer thickness value in order to prevent an adverse pressure gradient from happening, which then may lead to the formation of a flow separation. From experimental results, it is known that plasma actuator could decrease the thickness of the boundary layer by 9 mm.

Numerical simulation of vibration of horizontal cylinder induced by progressive waves

Maritime structures often comprise cylinders of small diameter relative to the prevailing wave length. This paper describes the direct forcing immersed boundary simulation of the hydroelastic behaviour of a rigid, horizontal circular cylinder in regular progressive waves. Fluid motions are numerically solved by the full Navier–Stokes equations, and the free surface by the volume-of-fluid method. The Reynolds number Re = 110, Keulegan–Carpenter number KC = 10, Froude number Fr = 0.69 and Ursell number U rs ≈ 12. A single-degree-of-freedom model is used for the elastically mounted cylinder. Velocity profiles for the stationary cylinder case have been successfully validated using experimental results. The frequency response for reduced velocities have been compared with theoretical data. Three transverse vibration regimes are identified: lower beating (); lock-in (); and upper beating () modes. The lower and upper beating regimes exhibit varying amplitude response. The lock-in mode represents the region of fixed and maximum response. The lower beating and lock-in modes have peaks at a common vibration to wave frequency ratio = 2. For the upper beating mode, = 1, except for when = 2.